During conventional continuous chemical pulp production, particularly in kraft cooking, the entire cooking liquor (e.g. white liquor) charge is added to the feed system, which includes the high pressure feeder and the circulation line to the top of the digester either with or without an impregnation vessel. However over the last decade two significant advances have taken place in continuous chemical pulp production technology which have changed this. First the MCC.TM. digesters, and method, developed by Kamyr, Inc. of Glens Falls, N.Y., added white liquor into a central recirculation loop within the digester. Subsequently, EMCC.RTM. digesters and processes, also developed by Kamyr, Inc., provided for introduction of white liquor into the bottom (wash) circulation loop. While these digesters and systems have been commercially successful because they enhance the quality of the pulp produced, one unexpected problem resulted from the introduction of the cooking liquor at multiple points, instead of the entire white liquor charge being added to the feed system.
According to the present invention, it has been determined that, in continuous digesting systems where a plurality of feed points for the cooking liquor are provided, since the volume of relatively cool cooking liquor supplied to the feed system is reduced, higher temperatures occur in the top circulation line, i.e. the line returning liquid separated from the chips in the top of the digester to the high pressure feeder (either with or without an impregnation vessel). This increases the potential for hydraulic hammering due to liquor flashing in the line, introducing loading on the adjacent equipment and piping, and providing potential hammering which can damage the high pressure feeder and adjacent piping and equipment. According to the present invention, the temperature of the liquid in the recirculation line, and circulation line, is kept low enough so as to avoid hydraulic hammering due to liquor flashing.
According to the present invention there is provided a method of feeding comminuted cellulosic fibrous material to a continuous digester having a plurality of feed points for cooking liquor, and utilizing a high pressure feeder. The method comprises the following steps: (a) Entraining comminuted cellulosic fibrous material in liquid to produce a slurry, and feeding the slurry to the top of the digester using the high pressure feeder. (b) Adding some cooking liquor to the slurry as part of the liquid entraining the material. (c) Separating some of the liquid from the slurry at the top of the digester. And, (d) recirculating the separated out liquid from the top of the digester to the high pressure feeder. According to the invention, the hydraulic hammering can be prevented when using one or both of the following techniques: the recirculating liquid can be cooled (by passing it into heat exchange relationship with a cooler liquid), and/or the cooking liquor may be cooled before it is added to the slurry (e.g. by flashing the cooking liquor to reduce its temperature, and produce flashed steam which may subsequently be used in an evaporator). If flashing of the cooking liquor is utilized, typically it is flashed when it has a temperature of about 90.degree. C., and the temperature thereof is reduced by at least about 10.degree. C., which can be enough--either singly or in combination with cooling of the recirculating liquid from the top of the digester--to avoid hydraulic hammering. The vacuum required for such flashing is provided through the connections made to the evaporator system.
According to another aspect of the present invention, a cellulosic pulp producing system is provided which comprises the following elements: A substantially upright continuous digester. A high pressure feeder. A circulating line operatively extending from the high pressure feeder to the top of the digester. A recirculating line operatively extending from the top of the digester to the high pressure feeder. A separator for separating liquid from a slurry containing cellulosic fibrous material and liquid, the separator disposed at the top of the digester and connected to the recirculating line. Means for adding cooking liquor to slurry being transported by the high pressure feeder to the top of the digester. And, heat exchanger means operatively disposed in the recirculating line for reducing the temperature of liquid being recirculated from the digester to the high pressure feeder.
The system may also comprise means for sensing the temperature of liquid in the recirculating line, means for regulating the flow rate of coolant to the heat exchanger means (e.g. a valve), and means for controlling the coolant flow rate regulating means in response to the temperature sensing. An impregnation vessel may be disposed in the recirculating and circulating lines between the high pressure feeder and the continuous digester, in which case the heat exchanger means is typically in the recirculating line between the impregnation vessel and the high pressure feeder.
According to yet another aspect of the present invention a cellulose pulp producing system is provided comprising: A substantially upright continuous digester. A high pressure feeder. A circulating line operatively extending from the high pressure feeder to the top of the digester. A recirculating line operatively extending from the top of the digester to the high pressure feeder. A separator for separating liquid from a slurry containing cellulosic fibrous material and liquid, the separator disposed at the top of the digester and connected to the recirculating line. Means for adding cooking liquor to slurry being transported by the high pressure feeder to the top of the digester. And, means for cooling the cooking liquor before supplying it to the means for adding cooking liquor. The cooking liquor cooling means preferably comprises a flash tank, including a steam discharge, and the steam discharge is operatively connected to evaporators.
It is the primary object of the present invention to provide a method and apparatus for avoiding hydraulic hammering or the like in modern continuous digesting systems in which a plurality of feed points for cooking liquor are provided. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended.